Method and apparatus for intelligent noise reduction in a distributed communication system

ABSTRACT

A wireless distribution system includes a number of remote units distributed in a coverage area to receive wireless signals and to provide the signals through the distribution system to input ports of an expansion unit where the signals are combined, detectors operatively connected to one or more of the plurality of input ports to determine the presence of information carrying signals at the input ports, a number of selection circuits to select signals in which information has been detected, and a node to combine the selected signals.

TECHNICAL FIELD

The present invention is related to a distributed communications system,and more particularly to a method and apparatus for noise reduction in adistributed communication system that may be implemented in amultipoint-to-point communication system in which wireless (radiofrequency or otherwise) signals are distributed in buildings or otherareas where wireless signal propagation is likely to be a problem.

BACKGROUND INFORMATION

The operational range of a system that combines and distributes signalswithin buildings or other areas where wireless signal propagation islikely to be a problem, whether the signals are digital or analog orsome combination of both, is limited by the dynamic range handlingcapability of the system. Background noise effectively limits thedynamic range of a system. In a distributed system, such as the onedisclosed in U.S. patent application Ser. No. 09/619,431,“Point-To-Point Digital Radio Frequency Transport,” filed on Jul. 19,2000 (Attorney Docket No. 100.01 9US01), incorporated herein byreference as if fully set forth, signals of varying levels as well asbackground noise, are present at input ports, at signal combiners and atoutput ports. When combining signals from various antennas and otherinput ports and remote units, with or without individual amplifiers, thenoise floor of the system generally increases by 10 log N, where N isthe number of signals being combined. The problem is exacerbated whensignals are combined from RF receivers that each have a given noisefigure and the combined signals may contain aliases, images and spurs ofthe background noise due to sampling effects, frequency translation andthe like. If such signals are combined in a purely additive way withoutregard to whether any useful information is present, the overall noiseperformance of the system will suffer. It would thus be desirable tointelligently select input signals so that overall system noise may bereduced.

The above-mentioned problems with noise performance in distributedcommunication systems within buildings and other enclosed areas, as wellas other problems, are addressed by the present invention and will beunderstood by reading and studying the following specification.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is an illustration of one example of a distributed intelligentnoise reduction system according to the teachings of the presentinvention.

FIG. 2 is a block diagram of one example of a distributed intelligentnoise reduction system according to the teachings of the presentinvention.

FIG. 3 is an illustration of one embodiment of a point-to-multipointcommunication system according to the teachings of the presentinvention.

SUMMARY

A wireless distribution system according to one aspect of the presentinvention includes a plurality of remote units distributed in a coveragearea to receive wireless signals in the coverage area and provide thesignals to a plurality of input ports to receive signals comprising thesignals provided by the plurality of remote units, a plurality ofintelligent detectors operatively connected to one or more of theplurality of input ports to intelligently determine whether signals atthe input ports carry any useful information, a plurality of selectioncircuits to select signals identified by the plurality of intelligentdetectors as carrying useful information, and a node to combine theplurality of selected signals.

DETAILED DESCRIPTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which are shown by way of illustration specific embodiments inwhich the invention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

FIG. 1 is an illustration of a representative system 100 fordistribution of wireless signals in a difficult environment for wirelesssignal propagation, such as in a large building or enclosure.Embodiments of the present invention are not limited to wireless signalpropagation in and around buildings or other enclosures but areapplicable to enhanced wireless systems for any appropriate coveragearea. While wireless systems typically provide bi-directionalcommunications, embodiments of the present invention provide enhancedupstream signal handling.

The system 100 includes a number of digital remote units (DRUs) 102 _(i)to 102 _(n) that receive a frequency spectrum of wireless signals suchas may be transmitted by low power wireless devices in and around thebuilding. In some embodiments, these include wireless signals incompliance with one or more of AMPS, PCS, GSM, TDMA, CDMA, and othersimilar wireless communication protocols. In general, the DRUs processthe spectrum of wireless signals so that the spectrum can be transmittedover transmission links 104 _(i) to 104 _(n), which may include one ormore transmission media such as fiber optic, coaxial, twisted pair orsimple copper wire, wireless link, or other medium of informationtransmission, and interface devices for such media or combinationsthereof. The signals may be processed before, during and aftertransmission over the transmission links 104 _(i) to 104 _(n) to improvesignal characteristics and propagation over the transmission media.Processing of the signals may include analog to digital conversion,analog and digital filtering, mixing and frequency translation,amplification and other well-known signal processing techniques. Thetransmission links 104 _(i) to 104 _(n) terminate at a node 110 ofdigital expansion unit (DEU) 101 where the signals are combined. Signalscombined at node 110 can originate from the various DRUs 102 as well asfrom other upstream nodes such as DEUs, which also combine signals fromDRUs or other DEUs.

One way to limit the total amount of system noise is to limit signalsthat are combined to those that actually carry useful information. Onesimple way to do this is to compare the incoming signal with a noisefloor threshold. Signals that are above the threshold likely containuseful information and are permitted to pass. Those that fall below thethreshold are unlikely to contain any useful information and are blockedso that they are not combined with other signals. In general, a useful,information carrying radio frequency (RF) signal, i.e., a signal that iscapable of being demodulated, will have a minimum signal to noise ratio(SNR). An information carrying signal may be detected by determiningwhether signal power or SNR within a certain frequency range exceeds apredefined threshold level for a sufficient duration of time.

The minimum SNR will depend on the type of modulation. For example, ahigher SNR will generally be needed to demodulate a carrier-basedmodulated signal than a non-carrier based modulated signal. For example,a non-carrier based spread spectrum signal (such as a code divisionmultiple access (CDMA) modulated signal) or an ultra-wide bandwidth(UWB) signal may be demodulated in the presence of a great deal of noiseand is often extremely difficult to distinguish from noise, particularlyfor ordinary narrowband receivers. Every CDMA modulated signal uses avery wide frequency spectrum, with separate encoding to identify thesignals. Each signal is modulated by a preselected pseudo-random orpseudo-noise (PN) sequence—direct sequence and frequency hopping are thetwo most common methods—that rapidly cycles the original signal throughmultiple individual narrowband slots. The resulting signals have theappearance of noise but may be demodulated with the appropriate PN key.Every time a CDMA signal is added to the system, the noise floor rises.But the amount of noise that each CDMA signal adds to the noise floor isnot constant. It is a function of the location of the CDMA transmitter,power output and the amount of information the CDMA signal is carrying,which may vary from moment to moment. Thus, as will be explained ingreater detail below, detecting the presence of some forms ofinformation-bearing signals may require processing and/or demodulation.

One example of a digital expansion unit (DEU) in a distributedcommunication system according to the present invention is shown inFIG. 1. DEU 101 includes a node 110 for summing upstream signalsreceived on transmission links 104 _(i) to 104 _(n) from DRUs 102 i to102 n. After the signals are combined at node 110, the combined signalsare fed to transmission link 105 which provides the combined signals toanother node or nodes in the system.

While the system of FIG. 1 shows only one DEU, it should be understoodthat many such DEUs may be included in a large distributed communicationsystem. For example, FIG. 3 shows a larger distributed communicationsystem 300 in a complex of buildings 302. As shown in FIG. 3,bi-directional transport of RF signals is accomplished through a networkof DRU's 304 which are positioned throughout the coverage area

System 300 includes a digital host unit (DHU) 306, which interfaces witha wireless network 305 via a wireless interface device (WID) 307. DHU306 is coupled to the public switched telephone network (PSTN), or amobile telecommunications switching office (MTSO) or other switchingoffice/network. DEU 308 is situated between the DHU 306 and one or moreDRUs. In the forward path, DEU 308 expands the coverage area bysplitting signals received from DHU 306 to a plurality of DRUs 304. Inthe reverse path, DEU 308 receives signals from a plurality of DRUs 304,sums the signals together and transports them to a DHU 306 or anotherDEU such as DEU 308. The system allows for successive branching ofsignals using DEUs 308 and expanded coverage to multiple DRUs 304.

FIG. 2 is an illustration of one example of a circuit for intelligentnoise reduction in a distributed communication system according to thepresent invention. System 200 includes control of the switching on andoff of upstream signals from components that are distributed in acoverage area of a multiple point to point wireless system. The system200 includes a number of digital remote units 202 _(i) to 202 _(n) thatreceive wireless signals and process them for delivery upstream overtransmission links 204 _(i) to 204 _(n). In general such transmissionlinks may be fiber optic, coaxial, twisted pair, wireless, or othermedium of information transmission, or combination thereof. Each DRU 202_(i) to 202 _(n) includes an input port or receiver that receives,processes and digitizes a wireless bandwidth. In one example each DRUreceives the same wireless bandwidth. The DRUs 202 _(i) to 202 _(n)transmit the digitized signals upstream to a digital expansion unit(DEU) or to a digital host unit (DHU) that includes summing junction226. Summing junction 226 sends the summed digitized signals to anotherDEU or a digital host unit 232 (DHU) located further upstream.

DEU 240 has a number of input ports 205 _(i) to 205 _(n) for receivingtransmission links 204 _(i) to 204 _(n) from each DRU or DEU locateddownstream from DEU 240. Each input port 205 _(i) to 205 _(n) hasassociated with it a detector 209 _(i) to 209 _(n) to determine whetherthe levels of signals received over transmission links 204 _(i) to 204_(n) exceed a predetermined threshold or otherwise match a pattern thatis characteristic of information carrying signals. Detectors 209 _(i) to209 _(n) may be operatively coupled to switches 215 i to 215 n so thattransmission links 204 i to 204 n are switched online or offline inresponse to the detection or absence of useful signals by the detectors209. In one example of the present invention the outputs of detectors209 i to 209 n may be used to directly operate the switches 215 i to 215n whenever a predetermined signal level threshold has been exceeded onan associated transmission link.

In many communication systems, it is valid to assume that signal levelsabove a predetermined threshold will contain useful information. Forexample, in the case of cellular signals, only licensed carriers andtheir subscribers have authorized use of this band. Therefore, it can beassumed that power detected above a certain threshold above the ambientnoise floor of the receiver is useful. Even if a remote unit is left offat the onset of a new signal, the signal will eventually increase as thecell phone attempts to contact the base station. This signal will exceedthe predetermined threshold of a nearby remote and will cause the DRUremote or the DEU to allow the signal to be combined.

The additional noise power as remote units are added to the system maybe expressed by the equationOverall system Noise Figure=NF+10LogN

-   -   where N is the number of remote units and NF is the noise figure        of a single remote in a complete end to end system.

The SNR threshold may be determined in various ways. For example, thethreshold may be determined during initial installation or during a lowusage time. Power thresholds may in general be computed as follows:

${Power} = {\frac{1}{N}{\sum\limits_{n = 0}^{N}X_{n}^{2}}}$where N is the number of samples and X are the samples. An ambient noisethreshold may then be compared with a signal threshold. N should be asufficient number of records to ensure a complete timeslot reuse.N=f _(s) T where f _(s) is the sample rate and 1 ms ≦T≦100 ms.

Once useful information has been detected, the signals are provided viatransmission links 204 i to 204 n to node 226 via transmission links 218_(i) to 218 _(n). Node 226 digitally sums the signals from transmissionlinks 218 _(i) to 218 _(n). The output signal fed to transmission link231 is the combined signals of 218 _(i) to 218 _(n).

In an alternative example of the present invention, signals fromdetectors 208 _(i) to 208 _(n) may be provided to controller/processor230 for processing to determine if useful information is present. Forexample, if spread spectrum signals are present in the coverage area, ina controller based intelligent detection system, the controller/signalprocessor may scan the RF spectrum of interest on each transmission linkto look for the presence of information carrying signals. A simplethreshold detector that indicates signals are present whenever the noisefloor rises above a predetermined level may be sufficient to detect thepresence of spread spectrum signals in some applications.

Alternatively, in applications where it is necessary to detect signalsthat may not cause a detectable rise in the noise floor, demodulationmay be employed as part of the detection process. Thus, as signals froma particular transmission link are selected, they may be downconvertedto baseband, despread, decoded, demodulated and otherwise processed todetermine if useful information is present.

In another example of the invention, information carrying signals may bedetected by multiplying or autocorrelating the signal with a delayedversion of itself followed by spectral analysis to detect the presenceof information. If the chip rate of the spread spectrum signals to bedetected are not known, it may be necessary to scan over a range ofdelays approximating the chip rates of signals that are likely to bepresent. Upon detecting useful information on a transmission link,controller 230 may provide a control signal to switch the appropriatetransmission link online. Controller 230 may be a dedicated controlleror signal processor physically located in DEU 240, or may be part of alarger system-wide central controller or processor. The process ofdetermining whether useful signals are present on transmission links 204i to 204 n and signal switches 215 i to 215 n must be updated at asufficient frequency so that useful information is not lost.

CONCLUSION

A distributed wireless intelligent noise reduction system has beendescribed. The distributed wireless intelligent noise reduction systemincludes a number of remote units distributed in a coverage area toreceive wireless signals in the coverage area, a number of input portsto receive signals from the remote units, input signal detectorsoperatively connected to each of the input ports to determine thepresence of information bearing signals received at each input port,selection circuits to select the information bearing signals, and a nodeto combine a plurality of the signals selected from the plurality ofinput ports.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. For example, DHUs andDEUs are not limited to the receipt, selection and summing, splittingand transmitting of digitized wireless signals. In some examples of thepresent invention, DHUs and DEUs are capable of receiving, processing,selecting and summing analog wireless signals in addition to or insteadof digitized wireless signals. As well, DHUs and DEUs may be capable ofsplitting, processing and transmitting analog wireless signals inaddition to or instead of digitized wireless signals. This applicationis intended to cover any adaptations or variations of the presentinvention. Therefore, it is intended that this invention be limited onlyby the claims and the equivalents thereof.

1. A wireless distribution system, comprising: a plurality of remoteunits distributed in a coverage area to receive wireless signals; aplurality of input ports to receive signals comprising the wirelesssignals provided by the plurality of remote units; a plurality ofdetectors operatively connected to one or more of the plurality of inputports to detect the presence of information carrying signals at theinput ports, a plurality of selection circuits to select the signals oninput ports identified by the plurality of intelligent detectors ascontaining information carrying signals, wherein the signals containinginformation are selected to limit the total amount of system noise; anode to combine the plurality of selected signals; and a transmissionlink to transmit the signals combined at the node to at least upstreamnode where the combined signals may be further combined with othersignals.
 2. A wireless distribution system, comprising: a plurality ofremote units distributed in a coverage area to receive wireless signals;a plurality of input ports to receive signals comprising the wirelesssignals provided by the plurality of remote units; a plurality ofdetectors operatively connected to one or more of the plurality of inputports to determine the presence or absence of information carryingsignals at the input ports; a plurality of switches to select one ormore of the signals received at the plurality of input ports in responseto signals from one or more of the plurality of detectors, wherein thesignals containing information are selected to limit the total amount ofsystem noise; a node to combine the signals selected from the pluralityof input ports; and a transmission link to transmit the signals combinedat the node to at least one upstream node where the combined signals maybe further combined with other signals.
 3. The wireless distributionsystem of claim 2 wherein the detectors comprise threshold detectors. 4.The wireless distribution system of claim 2, further comprising aprocessor to process signals from one or more of the detectors todetermine the presence of information.
 5. The wireless distributionsystem of claim 4 wherein the processor comprises a correlator.
 6. Thewireless distribution system of claim 2, wherein the signals received atthe input ports comprise a frequency spectrum that is digitized fordistribution over the wireless distribution system.
 7. The wirelessdistribution system of claim 6 wherein the digitized wireless spectrumis transmitted, at least in part, over a fiber optic transmission line.8. A method for controlling the signal levels of a wireless distributionsystem, the method comprising: receiving wireless signals at a pluralityof remote units distributed in a coverage area; providing signals fromthe remote units to a plurality of input ports; detecting the presenceof information carrying signals at the input ports, selecting thesignals on input ports containing information carrying signals, whereinthe signals containing information are selected to limit the totalamount of system noise; combining the plurality of selected signals; andtransmitting, on a transmission link, the combined signals to a leastone upstream node where the combined signals may be further combinedwith other signals.
 9. The method of claim 8 wherein detecting thepresence of information carrying signals at the input ports comprisesthresholding.
 10. A wireless distribution system, comprising: aplurality of remote units distributed in a coverage area to receivewireless signals and to provide the wireless signals through thedistribution system to one or more input ports; a plurality of detectorsoperatively connected to the one or more input ports to detect thepresence of information in signals received at the input port; aplurality of selection circuits to select signals in which informationhas been detected, wherein the signals containing information areselected to limit the total amount of system noise; a node to combinethe selected signal; and a transmission link to transmit the signalscombined at the node to at least one upstream node where the combinedsignals may be further combined with other signals.
 11. The wirelessdistribution system of claim 10 wherein one or more of the detectorscomprise a threshold detector.
 12. The wireless distribution system ofclaim 10 wherein one or more of the detectors comprise a correlator. 13.A digital expansion unit, comprising: a plurality of input ports toreceive signals from a plurality of remote units distributed in acoverage area to receive wireless signals; a plurality of detectorsoperatively connected to one or more of the plurality of input ports todetect the presence of information carrying signals at the input ports;a plurality of selection circuits to select the signals on input portsidentified by the plurality of intelligent detectors as containinginformation carrying signals; a node to combine the plurality ofselected signals; and a transmission link to transmit tile signalscombined at the node to at least one upstream node where the combinedsignals may be further combined with other signals.
 14. A digitalexpansion unit, comprising: a plurality of input ports to receivesignals from a plurality of remote units distributed in a coverage areato receive wireless signals; a plurality of detectors operativelyconnected to one or more of the plurality of input ports to determinethe presence of information carrying signals at the input ports; aplurality of switches to select one or more of the signals received atthe plurality of input ports response to signals from one or more of theplurality of detectors, wherein the signals containing information areselected to limit die total amount of system noise; end a node tocombine the signals selected from the plurality of input ports; and atransmission link to transmit the signals combined at the node to atleast one upstream nods where the combined signals may be furthercombined wit other signals.
 15. A method for controlling the signallevels of a digital expansion unit, the method comprising: receivingwireless signals at a plurality of input ports of a digital expansionunit; detecting the presence of information carrying signals at theinput ports; selecting the signals from input ports containinginformation carrying signals; combining the plurality of selectedsignals, wherein the signals containing information are selected tolimit the total amount of system noise; and transmitting on atransmission link, the combined signals to a least one upstream nodewhere the combined signals may be further combined with other signals.16. A method for controlling the signal levels of a digital expansionunit, the method comprising: receiving wireless signals at a pluralityof input ports of a digital expansion unit; measuring the strength ofsignals at the input ports; selecting signals front input ports having asignal strength that is higher than a predetermined threshold, whereinselecting these signals is to limit the total amount of system noise;combining the plurality of selected signals; and transmitting, on atransmission link, the combined signals to a least one upstream nodewhere the combined signals may be further combined with other signals.17. A wireless distribution system, comprising: a plurality of remoteunits distributed in a coverage area to receive wireless signals; aplurality of input ports to receive signals comprising the wirelesssignals provided by the plurality of remote units; a plurality ofdetectors operatively connected to one or more of the plurality of inputports to determine the presence or absence of information carryingsignals at the input ports, wherein the detectors comprise thresholddetectors; a plurality of switches to select one or more of the signalsreceived at the plurality of input ports in response to signals from oneor more of the plurality of detectors; a node to combine the signalsselected from the plurality of input ports: and a transmission link totransmit the signals combined at the node to at least one upstream nodewhere the combined signals maybe further combined with other signals.